A nonaqueous electrolyte battery using metallic lithium, a lithium alloy, a lithium compound, or a carbonaceous material in a negative electrode is expected as a high-energy density battery, and much research and development has been conducted. Lithium ion batteries each including a positive electrode containing LiCoO2 or LiMn2O4 as an active material and a negative electrode containing a carbonaceous material that allows lithium ions to be inserted/extracted have widely been put into practical use for portable devices so far.
On the other hand, when installing the battery in a vehicle such as an automobile or an electric train, materials with excellent chemical and electrochemical stability, strength, and corrosion resistance are needed as the materials of the positive and negative electrodes from the viewpoint of storage performance, cycle performance, high-output performance, and long-term reliability under a high-temperature environment (60° C. or more). If high performance is required in a cold district, high-output performance and long-life performance under a low-temperature environment (−40° C.) are needed. On the other hand, from the viewpoint of improving safety performance, nonvolatile incombustible nonaqueous electrolytic solutions have been developed. However, the solutions have not yet been put into practical use because they lower the output characteristic, low-temperature performance, and long-life performance.
As described above, when installing a lithium ion battery in a vehicle such as a car, there are problems of heat endurance and low-temperature output performance. It is therefore difficult to install and use a lithium ion battery in the engine room or an automobile in place of a lead storage battery.
The electrolytic solution of the lithium ion battery is used under a high voltage of 2 V to 4.5 V. For this reason, use of an aqueous electrolytic solution is difficult. A nonaqueous electrolytic solution formed by dissolving a lithium salt in an organic solvent is used as the electrolytic solution of the lithium ion battery. Conventionally, improving large-current discharge performance and cycle life performance by improving the nonaqueous electrolytic solution composition has been examined. A nonaqueous electrolytic solution can hardly reduce the resistance of a battery because the ionic conductivity is lower than that of an aqueous electrolytic solution. In addition, an organic solvent that is the solvent of the nonaqueous electrolytic solution lowers the high-temperature cycle life performance of a battery because it is readily decomposed at a high temperature, and the thermal stability is poor. For these reasons, use of a solid electrolyte as a nonaqueous electrolyte has been examined. However, since the ionic conductivity of the solid electrolyte is lower than that of the nonaqueous electrolyte, a battery having excellent large-current discharge performance cannot be obtained.